Credit instruments of the aforesaid character, such as so-called "debit" cards, are typically encoded with multiple identical data fields along the length of a strip or so-called "stripe" of recording media. This permits at least one of the data fields to be magnetically sensed and "read" for consummating a credit or other validation operation even if one data field is not readable, such as when data field is partly erased, or the magnetic media is scratched or otherwise damaged.
Such separation of the strip into separate data fields poses the risk the persons may authorizedly screen or cover portions of the strip, as by taping over them, to expose only one data field. The so-called screened card is then inserted into a credit issuing or validating mechanism to permit or cause a transaction to occur. Then, even though the exposed unscreened portion and its exposed data field may be cancelled, or rewritten, as with a lowered credit availability, the screened data fields still remain intact and may be used illegally to obtain additional transactions, such as in vending apparatus for which the card is designed. This is referred to as "jackpotting".
Such jackpotting by screening of a data field requires that the perpetrator determine in advance the locations on the strip at which data fields begin and end. In prior art magnetic encoding, it has often been the practice to divide the strip symmetrically about a center point into two distinct data fields. This makes jackpotting by a screening stratagem easy. A more sophisticated approach previously utilized is to separate the successive data fields by a series of "1's"or "0's" but without locating same symmetrically with respect to the card. Thus, the data may begin near one end of the strip and extend over only a portion of the strip.
However, prior magnetic encoding schemes, such as the "F-2" code and "Ratio" code, are such that magnetic "1's" and "0's" represented by magnetized regions each occupy the same interval along the data strip. Thus, in a binary coding scheme using only "1's" and "0's", each of two identical data field will be of an identical length. The would-be jackpotter then has to divide the total, or combined, length of the two data fields into two equal length halves being thereby given a good possibility that screening, such as by tape obscuration, of one half will permit a credit transaction to be consummated with the unobscured data field, thereby permitting a later jackpotting with the obscured data field.
In addition to being resistant to such jackpotting or unauthorized multiple use as described above, a magnetically encoded credit instrument such as especially a debit card must be resistant to errors in reading the magnetically encoded information thereon, such as produced by vibration and intermittent magnetic reading and writing causing abrupt, spurious variations in pulse time characteristics, termed "jitter", resulting from any uneven, but often unavoidable, movement of the card through a magnetic card reader. Such jitter is likely to result, for example, from intermittency of friction in a drive system which moves the card through the reader. Any change in the magnetic coding scheme to make it less prone to jackpotting carries with it the risk that the card or other credit instrument so coded will be more prone to reading or writing errors resulting from jitter.
In addition, an important constraint in the use of magnetic coding schemes is the effective bandwidth of electronic circuitry used for reading and writing the magnetically encoded data. If pulse widths or encoded data transitions become too short, as because of a poorly chosen coding scheme, then the bandwidth of the circuitry, as limited chiefly by its response time (in effect determining an upper frequency limit in a Fourier series transformation of the involved pulse waveforms), may be inadequate.